Fiber grating and fiber optic devices using the same

ABSTRACT

The present invention relates to a fiber grating which introduces a plurality of asymmetric microbends in a fiber. The present invention also relates to fiber optic devices, such as a fiber-optic filter, a fiber-optic polarizer, a fiber-optic wavelength tunable bandpass filter, a fiber-optic frequency shifter, using the above fiber grating which has asymmetric mode-coupling characteristics. The optical devices of the present invention exhibit a high mechanical durability and a long-term stability of the device, degradation of the optical fiber device due to a change in the characteristics of the grating can be prevented even after a long time at high temperature. In particular, the fiber grating according to the present invention has asymmetric mode coupling characteristics, so that it can be prevalently applied to an optical fiber notch filter, an optical fiber polarizer, an optical fiber wavelength tunable bandpass filter, an optical fiber frequency shifter and so on.

TECHNICAL FIELD

The present invention relates to a fiber optic element, morespecifically to a fiber grating that couples a light mode propagatingalong a fiber into another mode by a plurality of microbends formed inthe fiber.

The present invention also relates to optical devices, more specificallyto fiber optic devices, such as a fiber-optic filter, a fiber-opticpolarizer, a fiber-optic wavelength tunable bandpass filter, afiber-optic frequency shifter, using the above fiber grating which hasasymmetric mode-coupling characteristics.

BACKGROUND ART

Recently, increasing use is made of fiber Bragg gratings in variousfiber-optic applications such as telecommunications, fiber sensors andlasers. The fiber Bragg grating consists of a periodic stack of regionsof higher and lower refractive index along an optical fiber. Gratingsare made by exposing the core of a fiber to an interference pattern ofstrong laser light. It has the property of reflecting light within anarrow band of wavelengths and transmitting all wavelengths outside ofthat band. The central reflected wavelength is equal to twice the periodof the grating, multiplied by the fiber refractive index. For example, agrating reflecting at 1560 nm would have a period of about 535 nm. Mostof the fiber Bragg gratings have periods of a few 100 nanometers.

On the other hand, a long period fiber grating has a period of a few 100microns. The long period fiber grating couples a specific wavelengthlight, propagating along the core of the grating, into a cladding modeof the same propagating direction. The long period fiber grating can actas a band-rejection filter since the coupled cladding mode can easily bestripped. These long period fiber gratings have the advantages of easyfabricating, reduced fabricating cost and compact size. They willtherefore be useful in many applications including the gain-flatteningfilter of optical amplifiers.

Hereinafter, the conventional methods for fabricating these long periodfiber gratings will be explained in brief as follows:

[Method Using the Photosensitivity of Optical Fibers]

FIG. 1 shows the cross section of a conventional fiber grating that isfabricated using the photosensitivity of a single-mode optical fiber. Inprinciple, this method is the same as the conventional method forfabricating fiber Bragg gratings. However, this method should employ aspecific optical fiber including a fiber core with photosensitivityenhanced by doping therein Germanium(Ge) or the like.

Referring to FIG. 1, the side of a single-mode optical fiber is exposedto the light 10 of an excimer laser. The molecular structure of theexposed portions 30 in the fiber core 20 is deformed, thereby theportions 30 have higher refractive index. Thus, by irradiating the fiberwith uniformly spaced laser light along the fiber axis, a single-modefiber grating 40 with a periodically varying refractive index can beobtained. This grating couples a specific wavelength light, propagatingalong the core of the grating, into a cladding mode. Therefore, thisgrating can act as a filter.

FIG. 2 shows the cross section of another conventional fiber gratingthat is fabricated using the photosensitivity of a two-mode opticalfiber. The two-mode fiber grating 40′ is also fabricated by the samemanner as that of the single-mode fiber grating. The fiber grating 40′can couple the fundamental LP₀₁ mode into the second-order LP₁₁ mode,since the regions 30′ of higher refractive index are asymmetricallyformed along the fiber axis.

However, the fiber gratings fabricated by this method have adisadvantage that the gratings are erased with the passage of time. Inaddition, it is difficult to make shorter fiber gratings because theyhave low mode coupling efficiency.

[Method Using the Thermal Expansion of Fiber Core]

These fiber gratings are fabricated using the thermal diffusion of thedopants in the fiber core. When the core is strongly heated, the coreexpansion is induced by the thermal diffusion of the dopants.

FIG. 3 shows the procedure of fabricating such a fiber grating.Referring to FIG. 3, the core 22 of an optical fiber is locally heatedto form a core portion 24 with a larger radius by the light 12 from ahigh power laser. The light 22 is periodically scanned along the fiberaxis. For efficient local heating, a convex lens C focusing the light 12can be used together with the high power laser. Instead of the laserheating method, electric arc method may be used.

However, the fiber gratings fabricated by this method have adisadvantage that special optical fibers doped with an element of lowmolecular weight such as nitrogen should be used to enhance the thermalexpansion effect of the core.

[Method Using the Index Change Due to the Stress Removal]

In fabricating an optical fiber, if the fiber is cooled in a state thattensile force is applied to the fiber, stress will exist in the core ofthe fabricated fiber because of the difference of cooling speed betweenthe core and cladding. The stress can be removed by reheating the fiber,raising the refractive index of the core. Fiber gratings can befabricated using the above phenomenon. That is, heating an optical fiberlocally using a high power laser or an electric arc can induce therefractive index change.

However, this method should be applied to an optical fiber with a coremade of pure silica that is not doped with germanium or the like.

[Method Using the Periodic Deformations of Fiber Core]

It is well-known that closely spaced microbends in the fiber core, whichare introduced using two deformers with teeth thereon, can couple a coremode into a cladding mode or other core modes. In this case, thesymmetric core mode LP₀₁ can be coupled into asymmetric modes such asLP₁₁, LP₂₁ and LP₃₁ since asymmetric deformations are introduced alongthe fiber axis.

A schematic illustration of this fiber grating is shown in FIG. 4.Referring to FIG. 4, an optical fiber 60 is inserted between twodeformers 50 with periodic teeth thereon. The fiber 60 is bent to formmicrobends by the pressure F applied to the deformers 50. However, thefiber gratings fabricated by this method exhibit unstable performancecharacteristics depending upon the pressure applied to the deformers.

Another method was therefore suggested that could obtain betterstability in the periodic deformations. FIG. 5 shows the procedure ofintroducing periodic deformations in the fiber core by another method.Referring to FIG. 5, grooves G made by a CO₂ laser are spaced apart byan equal spacing. The grooves G are heated by the electric arc A ofelectrodes 70 vertically disposed on both sides of the optical fiber.The heated groove is melt to deform the fiber core due to surfacetension as shown in the left side of the electrodes 70. This method baseon the physical deformation are applicable to almost all types ofoptical fibers, but a high power laser is required to make grooves onthe fiber. Additionally, the grooves made on the fiber-weaken theoverall strength of the completed grating to resist torsion, bending andthe like loads. As described above, the conventional fiber gratings havethe disadvantages of poor characteristics and complexities in thefabrication process.

DISCLOSURE OF INVENTION

It is therefore an object of the present invention to provide animproved fiber grating which can be fabricated by simple process.

Another object is to provide a variety of improved optical devicesrealized by using the above fiber grating.

In order to accomplish the aforementioned object, the present inventionprovides a fiber grating for inducing a coupling between different lightmodes, comprising: a length of an optical fiber; and a plurality ofstepped microbends formed along the length of the optical fiber, each ofthe microbends being stress relieved.

The microbends may be spaced apart by a periodic distance substantiallyequal to a beat length of the different modes to be coupled and thenumber of the microbends may be preset to obtain a perfectmode-coupling. Otherwise, the microbends may be spaced apart bynonuniform distances.

The stress imposed by the microbends can be relieved to differentdegrees.

The stepped microbends preferably are formed by locally heating theoptical fiber in a state that mechanical stress due to force acting onthe side of the fiber is imposed on the fiber. More preferably, thelocal heating is carried out using an electric arc discharger, and mostpreferably, the microbends are heated with different arc intensity so asto relieve the stress to different degrees.

In order to accomplish another object, the present invention provides anoptical fiber device having a polarization-dependent mode-couplingratio, comprising: a length of an optical fiber havingpolarization-dependent effective refractive index; and a plurality ofstepped microbends formed along the length of the optical fiber. In thedevice, each of the microbends is stress relieved and the microbends arespaced apart by a periodic distance substantially equal only to a beatlength of two coupling modes for any one polarization component.Preferably, the optical fiber is a polarization maintaining opticalfiber or an elliptic core optical fiber. The device can further comprisea mode stripper for removing mode converted polarization component.

The optical devices which can be realized by the above fiber gratinginclude an optical fiber wavelength tunable bandpass filter comprising:an acoustic grating made by introducing a flexural acoustic wave into asingle mode fiber, the acoustic grating having predetermined wavelengthwidth and tunable center wavelength for a mode conversion of a passinglight; a fiber grating connected to the acoustic grating in series, thefiber grating inducing a mode coupling asymmetric to its own axis, thefiber grating having a mode conversion wavelength width broader thanthat of the acoustic grating; and a mode stripper for removing anasymmetric mode light passed through both the fiber grating and acousticgrating; wherein the band pass filter passes only light of thepredetermined mode conversion wavelength width at a desired wavelength.

The fiber grating used in the optical fiber wavelength tunable bandpassfilter may be the same as described above.

Another example of the optical devices which can be realized by theabove fiber grating is an optical fiber frequency shifter comprising: anacoustic grating made by introducing a flexural acoustic wave into asingle mode fiber, the acoustic grating producing both mode conversionand frequency shift for a passing light; and a fiber grating connectedto the acoustic grating in series, the fiber grating inducing a modecoupling asymmetric to its own axis so as to reconvert the modeconverted in the acoustic grating into its original mode withoutfrequency shift.

The fiber grating used in the optical fiber frequency shifter may alsobe the same as described above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view of a conventional fiber grating that isfabricated using the photosensitivity of a single-mode optical fiber;

FIG. 2 is a cross sectional view of another conventional fiber gratingthat is fabricated using the photosensitivity of a two-mode opticalfiber;

FIG. 3 shows the procedure of fabricating another conventional fibergrating using thermal expansion of a fiber core;

FIG. 4 shows a conventional method of introducing periodic deformationsin the fiber core;

FIG. 5 shows another conventional method of introducing periodicdeformations in the fiber core;

FIGS. 6A through 6C show a process for fabricating the fiber grating ofthe present invention;

FIG. 7 is a graph showing the filter spectrum of a long-period fibergrating fabricated using an optical communication grade single modefiber;

FIG. 8 is a graph of mode coupling ratio versus the wavelength of alight propagating through a dual-mode optical fiber mode converteraccording to the present invention;

FIG. 9 is a schematic view of a well-known acousto-optic optical fiberdevice; and

FIG. 10 is a schematic view of an optical fiber wavelength tunablebandpass filter realized by using both the acousto-optic optical fiberdevice of FIG. 9 and the fiber grating according to the presentinvention

BEST MODE FOR CARRYING OUT THE INVENTION

When the effective refractive index for a fiber core mode LP₀₁ isrepresented by n₀₁, and the effective refractive index for a modeLP_(mn) coupled to the core mode is represented by n_(mn), therelationship between the period Λ of a fiber grating and the centerwavelength λ of mode-conversion is shown as in the following Equation 1:$\begin{matrix}{\Lambda = \frac{\lambda}{n_{01} - n_{mn}}} & \left\lbrack {{Equation}\quad 1} \right\rbrack\end{matrix}$

Thus, the period of the grating suitable to a specific optical fiber, adesired mode, and a desired wavelength must be selected to induce adesired mode coupling. However, in the optical fiber grating accordingto the present invention, a core mode is coupled to only an asymmetricmode (e.g. LP₁₁, LP₂₁, LP₃₁, . . . ).

The fiber grating according to the present invention will now bedescribed referring to FIGS. 6A through 6C showing a process forfabricating the fiber grating of the present invention.

First, a suitable optical fiber is selected among various optical fiberssuch as a conventional communication grade optical fiber, an opticalfiber doped with a special material, a polarization maintaining opticalfiber, an elliptic core optical fiber, an elliptic cladding opticalfiber, a dispersion compensation optical fiber, a dispersion transitionoptical fiber, and a dual mode optical fiber. Next, the jacket of theoptical fiber is stripped, and the optical fiber is then fixed by twooptical fiber fixing boards.

Here, an induced stress generated by bending the optical fiber must notbe applied to the fixed optical fiber. If the stress exists, it shouldbe entirely removed by heating the optical fiber with torch flames.

As shown in FIG. 6A, one fixing board 110 is moved in parallel in adirection (T) perpendicular to the axis of a jacket-stripped opticalfiber 100, to thus induce a stress due to a step difference to theoptical fiber between the two fixing boards 110 and 112. Here, a stressdue to bending can be induced to the optical fiber by turning the twofixing boards 110 and 112 toward each other at a predetermined angle.

When an electrical arc is generated by applying a predetermined voltageto electrodes 120 vertically disposed on both sides of the optical fiber100, a portion of the optical fiber in contact with the electrical arcis melt to form a stepped microbend B on the optical fiber between thetwo fixing boards 110 and 112 by a stress due to a step difference asshown in FIG. 6B.

If an electrical arc is periodically generated by moving the electrodesin the lengthwise direction of the optical fiber, a fiber grating with agrating period of L can be completed as shown in FIG. 6C. The gratingperiod can be irregular to obtain a desired filter spectrum.

The microbends are periodically formed at the same spacing as the beatlength between different modes. Moreover, the microbends can be formedin a preset number to induce a perfect mode conversion between differentmodes.

The efficiencies of mode coupling at the microbends can be different bydifferently removing the stresses on the microbends. For this, it ispreferable that microbends are formed by locally heating the opticalfiber with electrical arcs of different intensity.

High performance optical devices, which can be achieved by adopting theabove asymmetric fiber grating, will now be described.

[Single-mode Optical Fiber Notch Filter]

FIG. 7 is a graph showing the filter spectrum of a long-period fibergrating fabricated using an optical communication grade single modefiber. The long-period fiber grating is completed by forming 75microbends on the single mode fiber at a period of 600 microns.Referring to FIG. 7, three notches at different wavelengths can beobserved, which are the results of optical losses that a core mode lightare entirely absorbed in a fiber jacket after being converted intodifferent cladding modes LP₁₂, LP₁₃ and LP₁₄. The center wavelength ofthe filter can be shifted by changing the period of the fiber grating.

[Dual-mode Optical Fiber Mode Converter]

FIG. 8 is a graph of mode coupling ratio versus the wavelength of alight propagating through a dual-mode optical fiber mode converteraccording to the present invention. The mode converter was fabricated byforming 13 microbends on the dual-mode optical fiber at a period of 515microns. The dual-mode optical fiber is a specific one that canpropagate core modes of LP₀₁ and LP₁₁ at 1300 nm while maintaining theirpolarizations. A mode conversion efficiency of 99% or more over awavelength range of 28 nm can be obtained around 1300 nm. The convertedLP₁₁ mode is not a cladding mode but a core mode, so that the light canpropagate in the optical fiber without loss. Since such a modeconversion within the dual-mode optical fiber can be absolutely madeonly by an asymmetric grating, a conventional symmetrical optical fibergrating is useless for this mode conversion.

[Optical Fiber Polarizer]

A polarization maintaining optical fiber is made to have differenteffective refractive indices for two polarizations of a core mode. Whenthe fiber grating according to the present invention is fabricated usingsuch a polarization maintaining optical fiber, and light is incidentupon the fabricated fiber grating, the mode conversion wavelength varieswith the polarization of the light as shown in Equation 1. Thus, aspecific wavelength region can allow mode conversion with respect toonly one polarization by equalizing the period of the polarizationmaintaining optical fiber grating to the beat length of two couplingmodes of the one polarization. However, the period of the polarizationmaintaining fiber grating must be greatly different from the beat lengthof two coupling modes of the other polarization. Therefore, the opticalfiber grating according to the present invention can be used as anoptical fiber polarizer by further comprising means for removing onlythe mode-converted polarization.

[Optical Fiber Wavelength Tunable Bandpass Filter and Frequency Shifter]

An acousto-optic optical fiber device, an optical fiber device forconstituting an optical fiber wavelength tunable bandpass filteraccording to the present invention, will now be described before theoptical fiber wavelength tunable bandpass filter. FIG. 9 is a schematicview of a well-known acousto-optic optical fiber device. Referring toFIG. 9, a first through hole is made through the cusp of a glass cone230, and second and third through holes are respectively made through athin cylindrical piezoelectric device 240 and a metal support board 250.The glass cone 230, the piezoelectric device 240, and the metal supportboard 250 are attached to each other so that the first, second, andthird through holes are aligned. A single-mode optical fiber 260 withits jacket stripped passes through these through holes. One surface ofthe piezoelectric device 240 contacts the flat surface of the glass cone230, and the other surface thereof is attached to the metal supportboard 250 by a conductive adhesive. The cusp of the glass cone 230 isalso attached to the optical fiber 260. An alternating voltage source270 is connected to both ends of the piezoelectric device 240 to applyan alternating voltage with a tunable frequency thereto. When amechanical vibration is generated in the piezoelectric device 240 byapplying the alternating voltage to both ends of the piezoelectricdevice 240, it is transmitted to the optical fiber 260 via the cusp ofthe glass cone 230, thus producing a wave (W) of microbends along theoptical fiber, i.e., an optical fiber acoustic grating. When the periodof this grating is equal to the beat length between two modes, e.g. LP₀₁and LP₁₁ modes, which can pass through the optical fiber 260, modeconversion occurs between the two modes. In order to produce microbendswith this specific period on the optical fiber, the piezoelectric device240 is driven with an alternating voltage with a specific frequencycorresponding to the microbends. While mode conversion occurs, thefrequency of light passing through the optical fiber is also shifted bythis specific frequency. When the propagation distance of a flexuralacoustic wave is controlled to be about 10 cm, the mode conversionwavelength width of the grating becomes several nanometers. In theacousto-optic optical fiber device using this acoustic grating, theperiod of the acoustic grating is easily controlled with the change ofthe alternating voltage frequency, so that the center of a modeconversion wavelength can also be easily controlled. Thus, this deviceis applicable to various optical devices such as a mode converter, awavelength tunable filter, a frequency shifter, and an optical switch,etc.

FIG. 10 is a schematic view of an optical fiber wavelength tunablebandpass filter realized by using both the acousto-optic optical fiberdevice of FIG. 9 and the fiber grating according to the presentinvention. Referring to FIG. 10, an acousto-optic optical fiber device300 and an optical fiber grating 310 according to the present inventionare connected to each other in series. The acousto-optic optical fiberdevice 300 has a certain predetermined wavelength width and tunablecenter wavelength for a mode conversion of an incident light. Theoptical fiber grating 310 connected to the acousto-optic optical fiberdevice 300 induces a mode coupling asymmetric to its own axis and has amode conversion wavelength width broader than that of the acousto-opticoptical fiber device 300. Thus, the passing light is almost entirelyconverted into a desired mode by the optical fiber grating 310, amongwhich only a light of a predetermined wavelength with about several nmwavelength width is converted into a LP₀₁ core mode by the acousto-opticoptical fiber device 300. Here, a bandpass-type filter spectrum can beobtained by adding means for removing light other than the LP₀₁ mode.Undoubtedly, the frequency of transmitted light increases or decreasesby the frequency of a flexural acoustic wave.

If the acousto-optic optical fiber device 300 and the optical fibergrating 310 according to the present invention has the same modeconversion wavelength width, the transmitted light is only frequencyshifted by the frequency of a flexural acoustic wave without modeconversion. Thus, this optical fiber wavelength tunable bandpass filtercan be used as an optical fiber frequency shifter. In this case, anadditional mode stripper is not necessary.

A conventional optical fiber wavelength tunable bandpass filter and afrequency shifter have been fabricated by using dual-mode opticalfibers, but those according to the present invention can be fabricatedby only single-mode optical fibers. Also, unlike the case of usingdual-mode optical fibers, polarization dependency of the devices issignificantly reduced.

As described above, the fiber grating and the optical fiber devicesusing the same are simply fabricated without requiring a special opticalfiber. Also, an entirely short optical fiber device can be fabricated onvirtue of the high mode coupling efficiency of the fiber grating.Furthermore, on account of a high mechanical durability and a long-termstability of the device, degradation of the optical fiber device due toa change in the characteristics of the grating can be prevented evenafter a long time at high temperature. In particular, the fiber gratingaccording to the present invention has asymmetric mode couplingcharacteristics, so that it can be prevalently applied to an opticalfiber notch filter, an optical fiber polarizer, an optical fiberwavelength tunable bandpass filter, an optical fiber frequency shifter,etc.

What is claimed is:
 1. A fiber grating for inducing a coupling betweendifferent light modes, comprising: a length of an optical fiber having afiber axis; and a plurality of microbends formed along the length of theoptical fiber, each of said microbends being stress relieved, whereinsaid plurality of microbends comprises plural steps formed along thelength of the optical fiber out of the fiber axis.
 2. The fiber gratingof claim 1, wherein said microbends are spaced apart by a periodicdistance substantially equal to a beat length of the different modes tobe coupled and the number of said microbends is preset to obtain aperfect mode-coupling.
 3. The fiber grating of claim 1, wherein saidmicrobends are spaced apart by nonuniform distances.
 4. The fibergrating of claim 1, wherein said microbends are stress relieved todifferent degrees.
 5. The fiber grating of claim 1, wherein said steppedmicrobends are formed by locally heating the optical fiber whileapplying mechanical stress on one side of the fiber in a directionperpendicular to the length of the fiber.
 6. The fiber grating of claim5, wherein the local heating is carried out using an electric arcdischarger.
 7. The fiber grating of claim 6, wherein said microbends areheated with different arc intensity so as to relieve the stress todifferent degrees.
 8. An optical fiber device having apolarization-dependent mode-coupling ratio, comprising: a length of anoptical fiber having polarization-dependent effective refractive indexand a fiber axis; and a plurality of microbends formed along the lengthof the optical fiber; wherein each of said microbends is stress relievedand said microbends are spaced apart by a periodic distancesubstantially equal only to a beat length of two coupling modes for anyone polarization component, wherein said plurality of microbendscomprises plural steps formed along the length of the optical fiber outof the fiber axis.
 9. The optical fiber device of claim 8, wherein saidoptical fiber is a polarization maintaining optical fiber or an ellipticcore optical fiber.
 10. The optical fiber device of claim 8, furthercomprising a mode stripper for removing mode converted polarizationcomponent.
 11. An optical fiber wavelength tunable bandpass filtercomprising: an acoustic grating made by introducing a flexural acousticwave into a single mode fiber, said acoustic grating havingpredetermined wavelength width and tunable center wavelength for a modeconversion of a passing light; a fiber grating connected to saidacoustic grating in series, said fiber grating inducing a mode couplingasymmetric to its own axis, said fiber grating having a mode conversionwavelength width broader than that of said acoustic grating; and a modestripper for removing an asymmetric mode light passed through both thefiber grating and acoustic grating; wherein said band pass filter passesonly light of said predetermined mode conversion wavelength width at adesired wavelength.
 12. The optical fiber wavelength tunable bandpassfilter of claim 11, wherein said fiber grating is comprised of: a lengthof an optical fiber; and a plurality of stepped microbends formed alongthe length of the optical fiber, each of said microbends being stressrelieved.
 13. An optical fiber frequency shifter comprising: an acousticgrating made by introducing a flexural acoustic wave into a single modefiber, said acoustic grating producing both mode conversion andfrequency shift for a passing light; and a fiber grating connected tosaid acoustic grating in series, said fiber grating inducing a modecoupling asymmetric to its own axis so as to reconvert the modeconverted in the acoustic grating into its original mode withoutfrequency shift.
 14. The optical fiber frequency shifter of claim 13,wherein said fiber grating is comprised of: a length of an opticalfiber; and a plurality of stepped microbends formed along the length ofthe optical fiber, each of said microbends being stress relieved.